Hydride phase dissolution enthalpy in neutron irradiated Zircaloy-4

The differential calorimetric technique has been applied to measure the dissolution enthalpy, ΔHδ→αirrad, of zirconium hydrides precipitated in structural components removed from the Argentine Atucha 1 PHWR nuclear power plant after 10.3 EFPY. An average value of ΔHδ→αirrad=5 kJ/mol H was obtained after the first calorimetric run. That value is seven times lower than the value of ΔHδ→α = 37.7 kJ/mol H recently determined in Zircaloy-4 specimens taken from similar unirradiated structural components using the same calorimetric technique [P. Vizcaı́no, PhD thesis, Number TD-11/03, Inst. Tech. 'Prof. Jorge A. Sábato', CICAC, CNEA, Buenos Aires, Argentina, 24 June 2003]. Post-irradiation thermal treatments gradually increase that low value towards the unirradiated value with increasing annealing temperature similar to that observed for TSSdirrad [P. Vizcaı́no, A.D. Banchik, J.P. Abriata. J. Nucl. Mater. 304/2-3 (2002) 96-106]. Therefore the same H atom trapping mechanism during reactor operation already proposed to explain the evolution of TSSdirrad is also valid for Qδ→αirrad. As the ratio Q/ΔH is proportional to the number NH of H atoms precipitated as hydrides, the increment of Qδ→αirrad with the thermal treatment indicates that the value of NH also grows with the annealing reaching the value corresponding to the bulk H concentration when ΔHδ→αirrad≈37 kJ/mol H. That is a direct indication that the post-irradiation thermal treatment releases the H atoms from their traps increasing the number of H atoms available to precipitate at the end of each calorimetric run and/or isothermal treatment.